In recent years, the sport of motocross racing has found increasingly wide acceptance in this country and others. While once primarily intended for motorcycle racing, motocross events for bicycles have become more common with the development of sanctioned tracks and independent bicycle motocross layouts. As is well known, motocross racing for either motorcycles or bicycles is conducted over courses having any number of sharp turns, bumps, jumps, grades, and other obstacles. It can be appreciated that every element of a cycle intended for motocross racing must be extremely durable.
It has been found that one area of particular stress concentration in motorcross bicycles is in the handlebar stem assembly which secures the handlebars to the frame. During normal racing of a motocross bicycle, or even in off-track use, the front wheel of the bicycle is alternately lifted from the ground by pulling the handlebars upwardly and then returned to the ground as the rider shifts his or her weight forward. Such motion occurs during racing as the bicycles encounter bumps or jumps, and in off-track use riders frequently do a "wheely" or ride on only the rear tire for short periods. The structure which secures the handlebars to the frame is thus subjected to repeated tensile and compressive loads, with the compressive loads being particularly high when the rider leans forward as the front wheel contacts the ground.
Traditionally, the handlebars of a bicycle have been connected to the frame by a simple bracket consisting of a tubular-shaped flange through which the handlebars are disposed and held in place by a single bolt. It can be appreciated that this construction proved to be completely unsatisfactory for motocross bicycles. In fact, to obtain the necessary strength, a four-bolt handlebar stem assembly was designed for use in such bicycles.
Generally, a handlebar stem assembly for motocross bicycles consists of a top plate and bottom plate which are essentially square in configuration and secured together by a bolt and nut connection at each of the four corners. The top and bottom plates are each formed with an arcuate groove through which the handlebar is disposed. The handlebar is secured in place by tightening the bolts which connect the top and bottom plates. Welded to the bottom plate is a tubular stem which extends downwardly at an angle into engagement with the frame of the bicycle for securing the handlebar stem assembly in place.
It is apparent that the alternating tensile and compressive forces created by lifting the front wheel from the ground and then lowering it back again are transmitted directly to the welded connection between the bottom plate and stem. In addition, tensile forces are applied to the welded connection between the bottom plate and stem as the bolts which secure the handlebar in place between the top and bottom plates are tightened. These bolts in effect impose a tensile pre-stress on the handlebar stem assembly which must also be accommodated in the connection between the bottom plate and stem.
It has been found that the tensile pre-stress applied to the handlebar stem assembly and the alternating tensile and compressive loads applied thereto during normal use of the bicycle create a substantial stress concentration at the welded connection between the bottom plate and stem at the inwardly extending edge of the stem where it forms an angle with the bottom plate. Fatigue cracks on the bottom plate along the welded connection to the stem may occur as a result of the stress concentration, leading to premature failure of the handlebar stem assembly.
To avoid this problem, several prior art solutions have been proposed. One approach has been to weld a gusset between the bottom plate and stem to relieve the stress concentration. Another solution to the problem found in the prior art is the use of a heat treated bottom plate which is welded to the stem. While both prior art designs appear to solve the problem of premature stress failure in the bottom plate, both do so at a cost which has been found to be excessive for many applications. In fact, it is believed that the expense required to produce a handlebar stem assembly using either a gusset or a heat treated bottom plate would be prohibitive for many applications if prices are to be competitive with foreign imports.
It has therefore been an object of this invention to provide a handlebar stem assembly which has acceptable fatique life and which may be manufactured for substantially less than guessets or heat treated stems.